In the wake of the ongoing deregulations of the electric power markets, load transmission and wheeling of power from distant generators to local consumers has become common practice. As a consequence of the competition between power producing companies and the emerging need to optimize assets, increased amounts of electric power are transmitted through the existing networks, frequently causing congestions due to transmission bottlenecks. Transmission bottlenecks are typically handled by introducing transfer limits on transmission interfaces. This improves system security.
However it also implies that more costly power production has to be connected while less costly production is disconnected from a power grid. Thus, transmission bottlenecks have a substantial cost to the society. If transfer limits are not respected, system security is degraded which may imply disconnection of a large number of customers or even complete blackouts in the event of credible contingencies.
The underlying physical cause of transmission bottlenecks is often related to the dynamics of the power system. A number of dynamic phenomena need to be avoided in order to certify sufficiently secure system operation, such as loss of synchronism, voltage collapse and growing electromechanical oscillations. In this regard, electrical power transmission systems are highly dynamic and require control and feedback to improve performance and increase transfer limits.
For instance in relation to unwanted electromechanical oscillations that occur in parts of the power transmission system, these oscillations generally have a frequency of less than a few Hz and are considered acceptable as long as they decay fast enough. They are initiated by e.g. normal changes in the system load or switching events in the network possibly following faults, and they are a characteristic of any power system. The above mentioned oscillations are also often called Inter-area modes of oscillation since they are typically caused by a group of machines in one geographical area of the system swinging against a group of machines in another geographical area of the system. Insufficiently damped oscillations may occur when the operating point of the power transmission system is changed, for example, due to a new distribution of power flows following a connection or disconnection of generators, loads and/or transmission lines. In these cases, an increase in the transmitted power of a few MW may make the difference between stable oscillations and unstable oscillations which have the potential to cause a system collapse or result in loss of synchronism, loss of interconnections and ultimately the inability to supply electric power to the customer. Appropriate monitoring and control of the power transmission system can help a network operator to accurately assess power transmission system states and avoid a total blackout by taking appropriate actions such as the connection of specially designed oscillation damping equipment.
It is known to dampen such interarea mode oscillations. Power oscillation damping is for instance described in “Application of FACTS Devices for Damping of Power System Oscillations”, by R. Sadikovic et al., proceedings of the Power Tech conference 2005, Jun. 27-30, St. Petersburg RU.
Damping may be based on local measurements of system properties, i.e. on system properties measured close to the location where the required damping is determined and also performed or be based on measurements in various areas of the system. The first type of damping is often denoted local power oscillation damping, while the latter case is normally termed wide area power oscillation damping.
The latter type of damping is in many ways preferred, since it considers the system performance globally and not locally. The measurements are in this case often collected using phasor measurement units (PMUs). However, since the measurements are collected from various areas of such a system, they may travel a long way before they reach a power oscillation damping unit where the wide area power oscillation damping is performed or controlled.
Because of this the measurement may be subject to delays in reaching the control equipment. This can be serious, because if the delay is too long it is possible that it may in some situations no longer be possible to dampen oscillations based on these measurements. Delay is furthermore only one type of deviation of a measurement that may have a negative influence on the damping. It can for instance also be faulty.
Most systems where power oscillations damping is performed also use a phasor data concentrator (PDC), which collects measurements and aligns them in time with each other and then forwards the aligned measurement according to the time of generation to the power oscillation damping unit. In doing this a PDC typically waits until all collected measurement values have been received that have a common time of generation, often in the form of a time stamp showing the time of generation. When all such time aligned measurements are received, the PDC then forwards them to the power oscillation damping unit. However, this means that if one measurement is delayed all will be delayed. This may therefore stop effective power oscillations damping.
There is therefore a need for enhancing the reliability of the control of power transmission systems and especially for improving power oscillation damping.
There exist some prior art in relation to power oscillation damping.
The article “Damping Controller Input-Signal Loss Effects on the Wide-Area Stability of an Interconnected Power System”, by Mekki et al in: Power Engineering Society Summer Meeting, 2000. IEEE, page 1015-1019, vol. 2, discusses the replacing of remote measurement signals in a Wide Area Control System with local measurements.
The article “Opportunities in Wide Area Control and Measurements (WACAM)” by Ghosh and Ledwich, Australasian Universities Power Engineering Conference AUPEC 2006, 10th-13 Dec. 2006, Melbourne, Victoria, Australia discusses using redundant remote measurements in a control design. Here a control parameter set is selected based on detection of a lost remote signal.
The article “Some Viewpoints and Experiences on Wide Area Measurement Systems and Wide Area Control Systems” by Xue, Power and Energy Society General Meeting—Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE, 20-24 Jul. 2008, page 1-6 discusses providing a unified information platform where monitoring systems and simulation systems are placed. Capability to analyze and control against blackouts is described as enhanced by using knowledge of extraction from PMU data and simulation results.
U.S. Pat. No. 7,149,637 discusses detection of oscillations and estimation of their parameters using a linear model.
The article “Wide-Area Monitoring and Control for Power System Grid Security”, by Avila-Rosales and Giri, 15th Power Systems Computation Conference Proc, 2005 discusses collecting PMU measurements with a PDC and using the measurements in an Energy Management System (EMS) application for predicting control actions.
PDCs are further known to be used in other areas than for control in a power transmission system. In US2008/0189061 PMUs and PDCs are used for calculating the sag of a power line.
However, none of these documents provide a solution to the above-mentioned problem. There is therefore still a need for improving reliability when performing power oscillations damping.
The present invention is therefore directed towards enhancing the reliability of the control of power transmission systems and especially for enhancing the reliability of power oscillation damping in a power transmission system.